Recent developments in the field of optical communication have lead to the more widespread use of monomode waveguides such as monomode optical fibres. It is particularly important in this case to devise a method for accurately positioning an optical component such as a laser chip or photosensor in alignment with an optical waveguide.
In accordance with one aspect of the present invention we provide a method of positioning an optical component and at least one optical waveguide in alignment with one another, the method comprising
(a) forming at least one elongate waveguide and a locating formation in a common substrate; and,
(b) mounting an optical component in the locating formation,
the relative positions of the locating formation and the or each waveguide being such that in use an optical beam may be coupled between the optical component and the or each waveguide.
By using such a locating groove, we have discovered that it is possible accurately to align a component with an optical waveguide since the component is located by the groove which is in the same single crystal as the waveguide. For example, a single crystal of silicon is a particularly suitable substrate for a number of reasons. These include the fact that silicon is readily available in large sizes to the most exacting standards of purity and perfection; photolithographic and etching techniques for defining surface and sub-surface geometries are highly developed; and most importantly large anisotropic etching rates exist between different crystallographic axes in the crystal. Furthermore, silicon has particularly useful electrical, mechanical and optical properties which enable the crystal to be used for opto-electronic sub-assemblies such as electro-optical modulators.
The invention also enables an optical component to be mounted in the same substrate as other optical devices.
The invention is particularly suitable for use with laser chips which have a line emission from an edge of the chip or with optical sensing chips. It has been very difficult up to now accurately to align the edge with an optical waveguide. However, these chips typically have a pair of angled supporting surfaces enabling the chip to be accurately positioned within the locating groove.
The method may further comprise causing optical radiation to be coupled between the optical component and the at least one waveguide, adjusting the position of the at least one or each waveguide, and monitoring the optical power coupled between the component and that at least one waveguide to determine the positions corresponding to maximum power coupling.
The optical component may typically be bonded in the locating groove by for example soldering.
The at least one optical waveguide may be formed by diffusing a suitable material into the substrate to change the refractive index of the substrate or by forming a substantially V-shaped groove in the substrate and mounting an optical fibre in the groove. Other methods are also possible.
Conveniently, in the former case, the locating groove is formed subsequently to the formation of an elongate optical waveguide, whereby the locating groove divides the elongate waveguide into two subsidiary waveguides. Alternatively, in the latter case the locating groove may be formed subsequently to the formation of a V-shaped fibre receiving groove, whereby the locating groove divides the V-shaped fibre receiving groove into two subsidiary V-shaped fibre receiving grooves, optical fibres being mounted in respective ones of the fibre receiving grooves.
With either of these methods, two optical waveguides are produced which are accurately aligned and, in the case where the optical component comprises a laser chip, may end adjacent opposite facets of the laser chip. This is useful for a number of reasons. Firstly, access to both facets helps to improve the spectral response of the laser chip. Secondly, a single laser chip can be used for both transmitting and receiving in optical communication systems; and thirdly monitoring of the laser chip can be carried out.
In accordance with a second aspect of the present invention, an optical assembly comprises an optical component and at least one optical waveguide which have been positioned in alignment with one another by a method in accordance with the first aspect of the invention.
In another arrangement, two optical waveguides are provided, adjacent ends of the waveguides being laterally offset, and wherein the locating groove intersects the adjacent ends of the waveguides such that the optical component positioned in the locating groove may be optically coupled with both waveguides.